The present invention relates to a toner binder resin for a dry-system developer for use in electrophotography, electrostatic printing, magnetic recording, etc., a toner containing the binder resin and processes for producing the binder resin and the toner.
Heretofore, as electrophotographic processes, a large number of processes have been known, including those disclosed in U.S. Pat. No. 2,297,691; and Japanese Patent Publication Nos. 23910/1967 and 24748/1968. These processes comprise the steps of forming an electrical latent image on a photosensitive member generally comprising a photoconductive material, subsequently developing the latent image with a toner, and optionally transferring the resultant toner image onto a transfer material such as paper and fixing the toner image by means of heat, pressure, solvent vapor, etc., thereby to obtain a copy. Where the step for transfering the toner image is included, there is also provided a step for removing residual toner.
Further, several developing methods have been known for visualizing electrical latent images, such as the magnetic brush method as disclosed in U.S. Pat. No. 2,874,063, the cascade developing method as disclosed in U.S. Pat. No. 2,618,552, the powder cloud method as disclosed in U.S. Pat. No. 2,221,776, and a method using an electroconductive magnetic toner as disclosed in U.S. Pat. No. 3,909,258.
The toner used in these developing methods conventionally comprises fine particles of a natural or synthetic resin and a dye or pigment dispersed therein. For example, fine particles with a size of the order of 1 to 30.mu. obtained by micropulverizing an intimate mixture of a binder resin such as polystyrene and a colorant dispersed therein, have been used as a toner. A magnetic toner is one containing particles of a magnetic material such as magnetite. In a system using a so-called two-component developer, a mixture of such a toner with carrier particles such as glass beads or iron powder is ordinarily used.
For the toner, although various physical and chemical properties are required, most known toners have a number of defects as explained hereinbelow. Thus, many of the toners readily fusible upon heating are liable to solidify or agglomerate during storage or in a copying machine. Many toners have poor triboelectric characteristics and poor free-flowability due to temperature change in the environment. Further, in the continuous use of many toners involving repetitive development, the density of the image is changed or the background density increases due to mutual deterioration of the toner, carrier particles and photosensitive member through collision between the toner particles and carrier particles, and contact between these particles and the photosensitive member. Further, many toners ordinarily cause background density to increase, resulting in so-called fogging, when it is actually intended to increase the density of copied images by increasing the amount of toner attached to the photosensitive member having a latent image.
One of these undesirable phenomena is caused by the fragility of a toner. That a toner is fragile means that the toner is readily pulverized by a mechanical power and is a desirable feature from the viewpoint of the productivity of a toner. However, a fragile toner is readily pulverized into fine powder even under a normal load applied thereto in a developing apparatus, thereby to cause undesirable phenomena such as fogging through contamination of carrier particles, soiling of a developer sleeve, and imperfect charge controlling characteristic of the toner particles per se. Thus, the fragility of a toner significantly affects the life of the developer. In order to obviate such deterioration, it is conceivable to use a polymer of a high molecular weight as a binder resin for the toner. However, use of such a high-molecular weight polymer is not desirable from the viewpoint of economization of energy consumption, because it requires a larger amount of heat due to elevation of a fixation temperature when the ordinary heat-fixation of images is carried out in the final step of the copying process. Further, in order to remove this drawback, it has been proposed to add a small amount of plasticizer into a toner, but such a proposal has not been necessarily successful because it is accompanied with problems such as decrease in free-flowability of the toner and contamination of the carrier particles. On the other hand, if a toner is too hard, practical production thereof becomes difficult because mechanical pulverization becomes nearly impossible.
For the above reasons, polystyrene, a styrenebutyl methacrylate copolymer or the like of a relatively low molecular weight of the order of several thousands having an appropriate hardness or rigidity has heretofore been used as a binder resin for toner. On the other hand, manufacturers are trying to produce copying machines and toners having decreased, or maintenance free, operation. It has been determined that, under these conditions, a relatively low molecular-weight polystyrene or styrene-butyl methacrylate copolymer is not sufficient in hardness, and a material with a higher hardness is required. Further, this class of ordinary binder resin is not especially suited for heat fixation by means of hot rollers which are the most widely adopted fixing means. More specifically, the binder resin is best adapted for giving a toner with good fixation characteristic, i.e., good adhesion thereof to a transfer medium such as paper through fusion of the binder on heating and pressure-application. However it is difficult to obviate the soiling of the heat rollers due to sticking of the toner onto the rollers, i.e., so-called offsetting. For this reason, silicone oil has been applied by adding a complicated mechanism, although such a measure is not completely satisfactory for removing offsetting and is disadvantageous from the points of cost as well as maintenance. Thus, development of a toner binder free of such a problem is desired. Several measures have been previously proposed, including changing binder components in various manners, various means such as crosslinking and others for adjusting the molecular weight of binder. There has also been adopted a measure of incorporating an additive such as a low-molecular weight polyolefin or another plasticizer into a toner. The incorporation of such an additive is, however, accompanied with various problems such as poor dispersibility with the binder, impairment of free flowability of the toner powder and the promotion of toner agglomeration, and a satisfactory additive has not been found so far.
As explained hereinabove, improvement of toner performances by the use of additives is limited, and improvement of the resin component which is a principal binder component is believed to be the most important. There have been some proposals of adjusting the molecular weight of a binder resin. These proposals are not more than saying that it is desired to broaden the molecular weight distribution by applying crosslinking and do not clarify what range of molecular weight distribution is desirable. The proposals made heretofore refer to a binder resin having a single peak in the molecular weight distribution and having an average molecular weight in a certain range, or to a binder resin further defined by a broadness of the molecular weight distribution in terms of a dispersion (more specifically, a ratio (Mw/Mn) of weight-average molecular weight (Mw)/number-average molecular weight (Mn) obtained by gel permeation chromatography (hereinafter, sometimes abbreviated as "GPC")). However, none of this category of binder polymers satisfies the overall requirements, particularly the overall fixation performances, including various characteristics as described hereinabove required for heat-fixable dry-system developers.
Further, binder resins having a molecular weight distribution satisfying certain relationships have been proposed. For example, Japanese Patent Laid-Open Applications Nos. 16144/1981 and 82258/1983 propose methods for improving the fixing characteristics of a toner by mixing a plurality of binder resins having different molecular weight ranges. Especially, Japanese Patent Laid-Open Application No. 82258/1983 discloses a binder having three peaks in the molecular weight distribution, which gives an improvement in the fixing characteristics. However, a binder resin only comprising three components of different molecular weights does not provide a satisfactory combination of fixability and anti-offset property on heat-fixation by means of heat rollers but still involves some problem in durability.